DE 197 48 556 A1 discloses a probe for eddy current measurement having ferromagnetic signal amplification, with the signal amplification being produced by a rigid ferritic core. Only test bodies with a planar surface can be measured by a probe which is formed from a rigid substrate on which planar coils are fitted. If the surfaces are uneven, the shape of the probe must be matched to a surface of the test body, otherwise, incorrect measurement values will be obtained.
A probe with eddy current measurement and with ferromagnetic signal amplification for planar test bodies is also known from U.S. Pat. No. 6,002,251.
U.S. Pat. No. 5,389,876 discloses a probe for eddy current measurement, although this produces only weak signals.